revert behavior of --format=FMT (-c)

revert behavior of --format=FMT (-c)
add new option: --printf=FMT

(isodigit, octtobin, hextobin): Define.
(PRINTF_OPTION): Define.
(interpret_backslash_escapes, trailing_delim): New globals.
(usage): Document them.  Alphabetize on long option names.
(print_esc_char): New function.
(print_it): Rewrite, in order to handle backslash escapes.
(main): Handle new option.  Set globals for --format, too.